Storage battery plate



United States Patent Office 2,771,500 Patented Nov. 20, 1956 STORAGEBATTERY PLATE Howard J. Strauss, Philadelphia, and Harold C. Riggs,

Langhorne Manor, Pa., assignors to The Electric Storage Battery Company,a corporation of New Jersey No Drawing. Application December 29, 1951,

Serial No. 264,281

1 Claim. Cl. 136-30) This invention relates to secondary or storagebatteries, particularly to the plates thereof and has for an object theprovision on the plates of a sheath or coating which increases the lifeof the battery without adversely affecting any electrical propertythereof.

' Though applicable to cells and batteries of other types, the presentinvention has been found to be particularly useful in connection withsilver-zinc-alkaline storage batteries in which shortened battery lifeis at least in,

part due to the failure of the diaphragms or separators to isolateplates of one polarity from those of the opposite polarity except forthe ionic flow of current therebetween. Inthe charging of such abattery, the metallic silver of the positive plate isgradually converteduntil when completely charged, the positive active material consists ofa mixture of silver oxide and silver peroxide, a mixture that isstrongly oxidizing in character.

In batteries of the foregoing type the diaphragm or separator has beenmade of regenerated cellulose, such as cellophane or fibrous sausagecasing, such diaphragm s or separators being susceptible to attack anddestruction by oxidation. A small amount of the positive active materialwill become migratory within the cell by dissolving in the electrolyteas complex ions or argentates, or by being colloidally dispersed in theelectrolyte. These particles or ions then come into contact with thediaphragm or separator and oxidize it while they themselves are beingreduced. The destruction of the separator or diaphragm by oxidation mayalso be accelerated by actual flaking oil of silver particles from thepositive plate. In either case, the particles lodging between thediaphragm and the positive plate or on the diaphragm form an extensionof the electrode surface toward and into contact with the diaphragm. Solong as they are in electrical contact with the positive plate theyundergo the same chemical changes as the positive active material, thatis to say, upon each recharge of the cell they are reconverted to silveroxide or silver peroxide and will then oxidize an additional portion ofthe diaphragm. As more particles are dislodged, the area of thediaphragm in direct physical contact with the mixture of silver oxideand silver peroxide is increased, and, therefore the destructiveoxidation of the diaphragm proceeds at an ever increasing rate.Unregenerated cellulose interposed between the diaphragm and thepositive plate, as in the form of porous paper slightly increases theinternal resistance and is itself subject to attack by oxidation. Theeffect in general of the addition of the porous paper is the provisionof a greater amount of material to be oxidized rather than offering asolution to the oxidizing problem itself.

In accordance with the present invention, it has been found that if thepositive plate be enclosed by a protect-- ing sheath or coating which isinert to the oxides and also of good non-conducting material, any silverparticles separating from the positive plate are insulated or removedfrom electrically conductive contact with the positive plate and,therefore, will no longer be changed in chemical composition duringcharging and discharging of the battery. The protecting sheath orcoating should not be of a material that itself has chemical reducingproperties; i. e., is subject to chemical change by oxidation. Thesheath must be inert to the strong alkaline electrolyte and it shouldnot materially increase the internal resistance of the cell.Furthermore, the sheath should preferably be of low cost and its mannerof application to each plate should lend itself to production methodswith good reproducibility as regards thickness and performance.

In a preferred form of the invention, a battery electrode or plate isprovided with the protective sheath or coating by dipping the plate intoa suspension of magnesium hydroxide (milk of magnesia). On air drying,the magnesium hydroxide which uniformly coats the surface of the plateis converted to a porous coating of magnesium oxide and magnesiumcarbonate. It has been found that if the plate first be dried, thesimple dipping operation followed by air drying results in a sheathwhich is relatively strong, and uniform in thickness throughout thesurface of the plate. It meets all of the necessary requirements abovespecified. Where.

the-thickness of the coating is to be increased, after the first coathas been dried and set by chemical change, the plate or plates may beagain dipped to build up the thickness and dried, and the process berepeated to attain the desired thickness. The sheath, after a single dipin the mangesia and after the setting thereof, appears to have athickness of from two-thousandths to threethousandths of an inch. Itaffects but little the internal resistance of the cell yet providescurrent interruption as between any particles of silver leaving thesurface area of the plate and hence, to a surprising degree, protectsthe diaphragm from destruction by oxidation.

The degree of improvement is indicated by an increased cell or batterylife of the order of one hundred percent of like cells or batteries thatdo not include the protective sheath of the present invention. In testscarried out on single-cell storage batteries of the samesilver-Zinc-alkaline type, in the absence of the present invention therewas attained a cycle life of from eight to ten cycles before cellsbecame unsuitable for continued use. The test conditions were asfollows: The cell was charged at a constant current of five amperes to atotal input of thirteen ampere hours (time required about 2.6 hours).After a rest period of from one to two hours following ch rge, each cellwas discharged at the rate of amperes for five minutes. If at the end ofthat time the battery voltage was one volt or over, the cell wasconsidered suitable for the next cycle of the test. The foregoingprocedure was continued for each cell until the battery voltagedecreased below one volt at the end of five minutes at which time thecell was considered unsuitable for further service. Cells, not includingthe present invention and tested as above, gave a cycle life of theabove indicated eight to ten cycles. Utilizing the same test procedure,cells having protective coatings on the plates as described above gavecycle lives of from sixteen to twenty cycles, although in some instancesas high as twenty-five cycles were obtained, with most cells givingeighteen cycles, As a result of the present invention, the cycle life ofthe battery has been greatly increased, in most cases more than doubled.

Magnesium hydroxide is the preferred material of which the sheath ismade not only because it meets all of the requirements outlined abovebut also for the reason that it is readily available at low cost as anemulsion in the form of milk of magnesia. Other materials having likeproperties, such for example as chromium hydroxide, can be used in placeof the milk of magnesia or magnesium hydroxide. Nickel hydroxide mayalso be used in place of the magnesium hydroxide or any metal hydroxidewhich, upon air drying or its equivalent, will change to a metal oxideand/or a metal carbonate, each of which is inert to the alkalineelectrolyte, and each of which is non-conductive to provide therequisite current-interrupting capability.

Where the thickness of the sheath is to be somewhat less than abouttwo-thousandths of an inch, the plate can be dampened with water. Thewetter the plate, the thinner will be the sheath finally formed thereonby a single dipping operation. To control wetness the plates areimmersed in water and then dried to a predetermined moisture content.Accurate control of the amount of magnesia which will adhere to theplates with each dipping operation is thereby achieved.

Air drying of each coat is preferred since it is necessary not only toremove the moisture from the magnesium hydroxide on the plate but alsoto provide conditions for conversion of the magnesium hydroxide tomagnesium oxide and, by reason of absorption of carbon dioxide from theair, the conversion of a part of the magnesium hydroxide to magnesiumcarbonate. The magnesium carbonate appears highly desirable for thereason that it imparts a greater toughness to the coating of the sheath.

After repeated charging and discharging of the battery to the end of theuseful life thereof, it has been found that the sheath or coating isrelatively free of silver and is still strongly mechanically bonded tothe plate.

It will be understood that the above described sheath is also usefulwhen applied to the negative plate. In this situation, the function ofthe sheath is to provide mechanical support for the negative material, afunction admirably carried out by reason of its inherent mechanicaltoughness and the bond formed between the sheath and the activematerial.

In copending application Serial No. 263,999, filed October 26, 1954, nowPatent No. 2,692,904, filed concurrently herewith there is disclosed abattery of increased cycle life attained by mixing a high molecularweight colloid dispersible in strong alkali electrolytes within theactive material of negative electrodes. Further in accordance with thepresent invention, the sheath or coating is applied to each positiveplate and such a colloid in amount not above about two percent by weightof the active material (zinc oxide) of each negative plate is thoroughlymixed therewith. Such a colloid may consist of one or more of thefollowing: gum arable, gum tragacanth, gum karaya, starch, licorice,lignin, and glues of animal origin. The presence of the colloid inhibitsthe crystalline growth of metallic zinc and prevents protuberances frompuncturing or rupturing the diaphragm. Gum arabic and starch arepreferred as the colloidal material with some preference being for theformer.

What is claimed is:

In a silver oxide-zinc battery containing an alkaline electrolyte, theimprovement consisting of a positive plate characterized by an enclosingsheath consisting of magnesium oxide and magnesium carbonate, saidsheath being sufliciently porous to permit the passage of electrolytetherethrough, and said sheath having been prodaced by dipping said platein an aqueous suspension of magnesium hydroxide, and air drying theplate so dipped.

References Cited in the file of this patent UNITED STATES PATENTS396,369 Roberts Jan. 15, 1889 623,195 Werner Apr. 18, 1899 668,356Placet Feb. 19, 1901 1,505,990 Willard Aug. 26, 1924 1,710,617 HaddonApr. 23, 1929 2,014,390 Lunn Sept. 17, 1935 2,594,711 Andre Apr. 29,1952 FOREIGN PATENTS 602,704 Great Britain June 1, 1948

